The present invention relates to a lithium battery and in particular to a lithium secondary battery with lithium and sulfur contained in a positive electrode thereof and a method of producing the lithium secondary battery.
Since the finding of conductive polyacetylene in 1971, conductive polymer electrodes have been studied vigorously as application of the conductive polymer for the electrode material may actualize development of improved electrochemical elements, such as batteries of light in weight and high energy density, large-area electrochromic elements, and biochemical sensors using minute electrodes. Relatively stable polymers like polyaniline, polypyrrole, polyacene, and polythiophene have also been developed as other xcfx80-electron conjugated conductive polymers. Lithium secondary batteries using these polymers as the positive electrode have also been developed. It is said these batteries have the energy density of 40 to 80 Wh/kg.
Organic disulfide has been proposed in U.S. Pat. No. 4,833,048 as the organic material that is expected to attain the higher energy density. The simplest form of the organic disulfide compound is represented as M+xe2x80x94xe2x88x92Sxe2x80x94Rxe2x80x94Sxe2x88x92xe2x80x94M+, where R is an aliphatic or aromatic organic group, S is sulfur, and M+ is a proton or a metal cation. This compound is combined via the Sxe2x80x94S bonding by electrolytic oxidation to form a polymer like:
M+xe2x80x94xe2x88x92Sxe2x80x94Rxe2x80x94Sxe2x80x94Sxe2x80x94Rxe2x80x94Sxe2x80x94Sxe2x80x94Rxe2x80x94Sxe2x88x92xe2x80x94M+
The polymer thus obtained is returned to the original monomer by electrolytic reduction. A metalxe2x80x94sulfur secondary battery, in which a metal M that supplies and traps the cation M+is combined with the organic disulfide compound, has also been proposed in the above US patent.
These batteries use the negative electrode of metal lithium and have the working voltage of 3 to 4 V and the energy density of not less than 150 Wh/kg. This level of energy density is comparable to or better than the energy density of conventional secondary batteries.
With a view to further increasing the energy density, the simple substance of sulfur may be used for the positive electrode by taking advantage of the high capacitance of sulfur as proposed in U.S. Pat. No. 5,523,179. This battery uses metal lithium for the negative electrode and is expected to have the working voltage of about 2V and the high energy density of 100 to 800 Wh/kg.
The organic disulfide compound, however, has a disadvantage of gradually decreasing the capacitance by repeated oxidation-reduction (charging-discharging), that is, polymerization through electrolytic oxidation (charging) and monomerization through electrolytic reduction (discharging). This is ascribed to the reason discussed below.
Oxidation (charging) of the organic disulfide compound gives a polydisulfide compound having electrical insulation properties and poor ion conductivity. The polydisulfide compound has a low solubility in an electrolyte. The organic disulfide monomer obtained by monomerization of the polydisulfide compound through reduction (discharging) has a high solubility in the electrolyte. The disulfide monomerized by reduction (discharging) is partly dissolved in the electrolyte, and the dissolved monomer is polymerized by oxidation (charging) at positions different from the original positions in the electrode. The polydisulfide compound polymerized and depositing apart from the conductive agent, such as carbon, is isolated from the electron-ion conductive network in the electrode and is not concerned in the electrode reactions. The repeated oxidation-reduction increases the isolated polysulfide compound and thereby gradually lowers the capacitance of the battery. The organic disulfide monomer having the high solubility easily moves to be escaped from the positive electrode into the separator, the electrolyte, and the negative electrode.
The battery having the positive electrode containing the organic disulfide compound accordingly has disadvantages of the lowered charge-discharge efficiency and the shortened charge-discharge cycle life. In the case where metal lithium is used for the negative electrode in order to improve the volumetric energy density, dendrite may be produced on the surface of the metal lithium in the course of the charge-discharge cycle. This may lead to a short circuit to make a fire or may lower the charge-discharge efficiency.
In the case where the simple substance of sulfur is used for the positive electrode in order to improve the capacitance, the battery has a relatively low working voltage of about 2V and poor charge-discharge cycle characteristics. The deterioration of the charge-discharge cycle in the case of using the simple substance of sulfur for the positive electrode may be ascribed to the phenomenon discussed below.
The simple substance of sulfur has lots of known allotropic forms. Cyclooctasulfur (S8) includes xcex1-sulfur (orthorhombic sulfur), xcex2-sulfur (monoclinic sulfur), and xcex3-sulfur (monoclinic sulfur). Cyclohexasulfur (S6) generally has a rhombohedral crystal system, but also includes macro-cyclic, chain, and amorphous structures. There is little energy difference among these structures. It is accordingly thought that sulfur forms chains and takes a variety of shapes unstably. Using the sulfur powder as a starting material of charge and discharge and repeating the charge (insertion of lithium) and discharge (removal of lithium) causes the sulfur of the initial state to be not reproduced and leads to isolation of the dielectric sulfur. This prevents the transmission of electrons and insertion of lithium ions.
One object of the present invention is thus to provide a positive electrode, where the redox reaction proceeds at a high speed even at room temperature and the voltage characteristics of the organic sulfide compound and the high capacitance and the high energy density of sulfur and lithium sulfide are not damaged.
Another object of the present invention is to provide a lithium secondary battery of high energy density, which suppresses escape and isolation of the positive electrode active material from the positive electrode during the charging and discharging operations, keeps the voltage characteristics of the organic sulfide compound and the high capacitance of sulfur and lithium sulfide, and has a relatively fixed discharge voltage of 3 V level.
Still another object of the present invention is to provide a method of manufacturing the positive electrode to provide such a lithium secondary battery.
The present invention is directed to a positive electrode, which contains at least one organic sulfide compound selected from the group consisting of a thiolato compound and a thiol compound, and at least one selected from the group consisting of sulfur and a lithium sulfide represented by Formula (1) given below:
(LixS)nxe2x80x83xe2x80x83(1)
where 0 less than xxe2x89xa62 and n greater than 0.
It is especially preferable that the positive electrode contains at least one organic sulfide compound selected from the group consisting of a thiolato compound and a thiol compound, and a lithium sulfide represented by Formula (1) given below:
(LixS)nxe2x80x83xe2x80x83(1)
where 0 less than xxe2x89xa62 and n greater than 0.
It is preferable that the organic sulfide compound is a metal complex.
In accordance with one preferable application of the present invention, the positive electrode further contains a conductive polymer.
It is preferable that the thiolato compound is at least one selected from the group consisting of a lithium disulfide compound, a lithium trisulfide compound, and a lithium tetrasulfide compound.
It is also preferable that the thiol compound is at least one selected from the group consisting of a disulfide compound, a trisulfide compound, and a tetrasulfide compound.
The present invention is also directed to a method of producing a positive electrode. The method includes the steps of: (a) dissolving at least one organic sulfide-compound selected from the group consisting of a thiolato compound and a thiol compound, in a solvent to provide a solution; (b) mixing at least one selected from the group consisting of sulfur and a lithium sulfide represented by Formula (1) given below with the solution thus obtained to provide a mixture:
(LixS)nxe2x80x83xe2x80x83(1)
where 0 less than xxe2x89xa62 and n greater than 0; and (c) applying the mixture thus obtained on a conductive substrate and heating the conductive substrate in an atmosphere of inert gas or in vacuo to obtain the positive electrode.
It is preferable that the solvent is at least one selected from the group consisting of tetrahydrofuran, N,N-dimethylformamide, and Nxe2x80x94R-2-pyrrolidone, wherein R represents either a hydrogen atom or an alkyl group.
In accordance with one preferable application of the present invention, the method further includes the step of adding a conductive polymer to the solution obtained in the step (a), prior to the step (b).
In accordance with another preferable application of the present invention, the method further includes the step of lithiating the thiol compound and/or sulfur included in the positive electrode, after the step (c).
The present invention is further directed to a lithium battery including the positive electrode discussed above, a negative electrode, and a non-aqueous electrolyte.
Further, the lithium battery may be a primary battery or a secondary battery, and the non-aqueous electrolyte includes a lithium salt.